Antibody 2G12, isolated from an HIV positive individual, binds and neutralizes a broad range of HIV strains (Trkola et al., J. Virol. 70:1100-1108 (1996) and Binley et al., J. Virol. 78:13232-13252 (2004)) and provides sterilizing immunity against SHIV challenge in macaque models of infection (Mascola et al., Nat. Med. 6:207-210 (2000); Hessell et al., Nat. Med. 15:951-954 (2009); and Hessel et al., PLoS Pathog. 5:e1000433 (2009)). 2G12 recognizes an epitope comprised of 2-4 high mannose (Man9GlcNAc2) glycans on the surface of HIV envelope protein gp120 (Scanlan et al., J. Virol. 76:7306-7321 (2002); Calarese et al., Science 300:2065-2071 (2003); and Calarese et al. Proc. Natl. Acad. Sci. U.S.A. 102:13372-13377 (2005)) and glycopeptides which precisely mimic this glycan clustering and presentation may be useful as vaccines to “re-elicit” 2G12-like antibodies in vivo (Scanlan et al., Nature 446:1038-1045 (2007)). Glycans clustered on carbohydrate scaffolds (Ni et al., Bioconjugate Chem. 17:493-500 (2006)), peptide scaffolds (Joyce et al., Proc. Natl. Acad. Sci. U.S.A. 105:15684-15689 (2008)), and protein scaffolds (Astronomo et al., J. Virol. 82:6359-6368 (2008)) as well as phage particles (Astronomo et al., Chem. Biol. 17:357-370 (2010)) and yeast (Luallen et al., J. Virol. 82:6447-6457 (2008); Luallen et al., J. Virol. 83:4861-4870 (2009); Agrawal-Gamse et al., J. Virol. 85:470-480 (2011); Ciobanu et al., Chem. Commun. 47:9321-9323 (2011); and Marradi et al., J. Mol. Biol. 410:798-810 (2011)) have been tested for this purpose, but with little success. In part, this may be due to the difficulty of designing structures in which the clustering of glycans faithfully mimics that of the 2G12 epitope on gp120. Indeed, most of these structures were recognized by 2G12 with orders of magnitude weaker affinity than was gp120, suggesting that they were not optimal mimics of the 2G12 epitope.
The directed evolution of glycopeptides has been of interest, given their relevance in both HIV and cancer vaccine design. Although many powerful methods are available for in vitro selection of peptides, comparatively little has yet been published on in vitro selection of glycopeptides. Recently phage display with chemically-modified phages enabled selection of peptide 5-mer sequences containing a single central mannose monosaccharide from ˜106 sequences (Arai et al., Bioorg. Med. Chem. Lett. 23:4940-4943 (2013)). In an alternative approach, a single mannose was chemically attached to the N-terminal position of a 7-mer phage-displayed library of ˜108 sequences, although selections with this library have not yet been reported (Ng et al., ACS Chem. Biol. 7:1482-1487 (2012)). Because carbohydrate epitopes of various pathogen (e.g., HIV) and cancer cells may contain multiple glycans (see Scanlan et al., J. Virol. 76:7306-7321 (2002); Calarese et al., Science 300:2065-2071 (2003); and Calarese et al. Proc. Natl. Acad. Sci. U.S.A. 102:13372-13377 (2005)), it is desirable that a selection method allow access to multivalent glycopeptides containing one or more glycans at variable positions, supported by a peptide framework.
More importantly, it is desirable for selected glycopeptides to exhibit high affinity binding to known carbohydrate-binding monoclonal antibodies or other targets used during selection. Such carbohydrate-binding monoclonal antibodies include antibodies known to neutralize pathogens and antibodies known to afford protection (i.e., cytotoxicity) against cancer cells.
The present invention is directed to overcoming these and other deficiencies in the art.